1. Field of the Invention
This invention relates in general to apparatus for modifying the time base of signals in a train thereof; more particularly, the invention is concerned with apparatus for correcting a time base instability in TV-type signals.
2. Description Relative to the Prior Art
While the invention is not so restricted, its utility as a flutter compensator in a video tape recorder dramatically indicates its significance. The playback of a taped video recording requires faithful replay of recorded signals without causing such signals to shift unwantedly and continuously in frequency. Unwanted frequency modulation of playback video signals can, among other things, cause (1) the sync information content of the video signal to become unstable, and (2) the color information content of the video to become, at best, faulty, i.e., color information may not be detectable at all, since unstable playback of recorded color burst information within the video signal may cause the color content of the video signal to be lost entirely.
The degree of unwanted frequency shift during playback of the video signal is measured in terms of "percent flutter," it being recognized that even fractional percent flutter will prevent the AFC circuits of most TV's from tracking the fluttering sync signal, resulting in the tearing apart of the visual display that corresponds to the playback signal. Percent flutter may be defined as 100 times a change in frequency divided by a nominal frequency, the nominal frequency in the present case being 15,750 Hz.
To assure against instantaneous time base instability of played back TV signals which are derived from a video tape recorder, present practice is to employ extremely close tolerances in the precision parts of the recorder and to complement such tolerances with various servo controls which actively prevent flutter from occurring during playback. Such practices greatly add to the cost of manufacturing video tape recorders and, indeed, have prevented video recorders from being cost-wise within reach of many who would otherwise want them. This being the case, various proposals have been made to utilize variable delay lines, e.g., U.S. Pat. Nos. 2,960,568; 3,238,300; and 3,580,991, as a way to nullify flutter within playback signals: flutter within the played back signals is detected; the "flutter" signal is then used to vary the delay of a delay line through which the fluttering signal is passed, thereby causing the output of the delay line to be, hopefully, a flutter-free equivalent of the fluttering input to the delay line.
Copending U.S. application Ser. No. 432,363 discloses a departure from the above-noted delay line concept, having, at the heart thereof, an analog shift register. Basically, copending Ser. No. 432,363 teaches (1) the clocking (sampling) of a fluttering signal into an analog shift register at a rate dependent on the degree of flutter, and (2) the clocking of such signal samples out of such analog shift register at a desired rate. If the desired rate is constant, flutter is nullified. On the other hand, the clock-out rate may be a variable, whereby the signal output of the analog shift register may be modified according to choice.
In utilizing the teaching of Ser. No. 432,363 to process signals such as TV type signals, i.e., signals which are continuous and which have a nominal periodicity, plural analog shift registers, say two, are used and so cooperate that while one TV line signal is clocked into one register at a flutter-dependent rate, the signal within the other register is clocked out of such register at a known, but stable, rate, and vice versa.
As is known, analog shift registers may take a variety of forms such, for example, as charge coupled and charge injection devices, various serially disposed sample-and-hold circuits, and even A/D and D/A converters which functionally sandwich digital shift registers.